The air stone, located at the base of the aquarium, breaks up the flow of air into the aquarium into a mass of bubbles. After a time the air stone can become blocked by calcification, especially in hard water areas, reducing the flow of air into the aquarium. Replacing the air stone every few months will keep your aquarium bubbling away nicely. In some hard water areas it may be necessary to replace the airstone more frequently, but once every 6 months should be sufficient for most owners. Replacing the airstone is extremely easy. The airstone is located in the centre of the aquarium under the filter cartridge, simply remove the filter cartridge and you will be able to locate the airstone. A replacement airstone can simply be pushed into place. Each BiOrb Air Stone accessory pack includes one relacement air stone.